The present invention relates generally to filters for electrical signals, more particularly to control of cross-coupling in narrowband filters, and still more particularly to methods and apparatus to control the placement of transmission zeroes when introducing cross-coupling between non-adjacent resonators in a narrowband filter.
Narrowband filters are particularly useful in the communications industry and particularly for wireless communications systems which utilize microwave signals. At times, wireless communications have two or more service providers operating on separate bands within the same geographical area. In such instances, it is essential that the signals from one provider do not interfere with the signals of the other provider(s). At the same time, the signal throughput within the allocated frequency range should have a very small loss.
Within a single provider""s allocated frequency, it is desirable for the communication system to be able to handle multiple signals. Several such systems are available, including frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and broad-band CDMA (b-CDMA). Providers using the first two methods of multiple access need filters to divide their allocated frequencies in the multiple bands. Alternatively, CDMA operators might also gain an advantage from dividing the frequency range into bands. In such cases, the narrower the bandwidth of the filter, the closer together one may place the channels. Thus, efforts have been previously made to construct very narrow bandpass filters, preferably with a fractional-band width of less than 0.05%.
An additional consideration for electrical signal filters is overall size. For example, with the development of wireless communication technology, the cell size (e.g., the area within which a single base station operates) will get much smallerxe2x80x94perhaps covering only a block or even a building. As a result, base station providers will need to buy or lease space for the stations. Since each station requires many separate filters, the size of the filter becomes increasingly important in such an environment. It is, therefore, desirable to minimize filter size while realizing a filter with very narrow fractional-bandwidth and high quality factor Q. In the past, however, several factors have limited attempts to reduce the filter size.
For example, in narrowband filter designs, achieving weak coupling is a challenge. Filter designs in a microstrip configuration are easily fabricated. However, very narrow bandwidth microstrip filters have not been realized because coupling between the resonators decays only slowly as a function of element separation. Attempts to reduce fractional-bandwidth in a microstrip configuration using selective coupling techniques have met with only limited success. The narrowest fractional-bandwidth reported to date in a microstrip configuration was 0.6%. Realization of weak coupling by element separation is ultimately limited by the feedthrough level of the microstrip circuit.
Two other approaches have been considered for very-narrow-bandwidth filters. First, cavity type filters may be used. However, such filters are usually quite large. Second, filters in stripline configurations may be used, but such devices are usually hard to package. Therefore, by utilizing either of these two types of devices there is an inevitable increase in the final system size, complexity and the engineering cost.
If a quasi-elliptical filter response is desired, it will be appreciated that transmission zeroes on both sides of the passband may be used to enhance the filter skirt rejections. For fewer poles and less Q requirements, a quasi-elliptical filter can achieve similar skirt rejections compared to a Chebyshev filter. FIG. 5a illustrates a simulated response of a 12-pole quasi-elliptical filter compared to a Chebyshev filter.
One method of achieving a quasi-elliptical filter response is to introduce a cross-coupling between two or more specific non-adjacent resonators. In microstrip filter designs, the separation(s) of non-adjacent resonators and the dielectric properties of the substrate determine the strength of the cross-coupling. If the layout topology of the filter is constructed such that desired non-adjacent resonators are close together, then the cross-coupling of such non-adjacent resonators can introduce transmission zeroes on both sides of the filter transmission. This results in the layout providing a beneficial parasitic effect in the quasi-elliptical filter response.
However, in the past the introduction of such non-adjacent cross-coupling has not been easily controlled. For example, depending upon the required filter size, number of poles and substrate choice, the transmission zeroes may not be provided at the appropriate location. Thus, at times the cross-coupling may not be large enoughxe2x80x94such that the transmission zeroes are at very low levels. At other times, the cross-coupling is too large, such that the transmission zeroes are at very high levelxe2x80x94which interferes with passband performance.
Therefore, there exists a need for a super-narrow-bandwidth filter having the convenient fabrication advantage of microstrip filters while achieving, in a small filter, the appropriate non-adjacent cross-coupling necessary to introduce transmission zeroes which provides an optimized transmission response of the filter.
The present invention provides for a method and apparatus to control non-adjacent cross-coupling in a micro-strip filter. In instances of weak cross-coupling, such as a filter circuit on a high dielectric constant substrate material (e.g., LaAlO3 with dielectric constant of 24), a closed loop is used to inductively enhance the cross-coupling. The closed loop increases the transmission zero levels. For strong cross-coupling cases, such as a filter circuit on a lower dielectric constant substrate material (e.g., MgO with dielectric constant of 9.6), a capacitive cross-coupling cancellation mechanism is introduced to reduce the cross-coupling. In the latter instance, the transmission zero levels are moved down.
In the preferred embodiment, the present invention is used in connection with a super-narrow band filter using frequency dependent L-C components (such as are described in Zhang, et al. U.S. Ser. No. 08/706,974 which is hereby incorporated herein and made a part hereof by reference). The filter utilizes a frequency dependent L-C circuit with a positive slope k for the inductor values as a function of frequency. The positive k value allows the realization of a very narrow-band filter. Although this filter environment and its topology is used to describe the present invention, such environment is used by way of example, and the invention might be utilized in other environments (for example, other filter devices with non-adjacent resonator devices, such as lumped element quasi-elliptical filters). Further, the environments of communications and wireless technology are used herein by way of example. The principles of the present invention may be employed in other environments as well. Accordingly, the present invention should not be construed as limited by such examples.
As noted above, there have been previous attempts to utilize non-adjacent parasitic coupling to introduce transmission zeroes in filters. However, such efforts have generally been provided purely as a parasitic effect without control. One example of such an attempt is described in S. Ye and R. R. Mansour, DESIGN OF MANIFOLD-COUPLED MULTIPLEXERS USING SUPERCONDUCTIVE LUMPED ELEMENT FILTERS, p. 191, IEEE MTT-S Digest (1994). Still other techniques have been developed to artificially add non-adjacent cross-couplings. Here the efforts have generally introduced transmission zeroes using a properly phased transmission line. Examples of these latter efforts may be found in S. J. Hedges and R. G. Humphreys, EXTRACTED POLE PLANAR ELLIPTICAL FUNCTION FILTERS, p. 97; and U.S. Pat. No. 5,616,539, issued to Hey-Shipton et al. None of these efforts, however, provide the precise cross-coupling control and flexibility to optimize the filter performance.
Referring more specifically to the device disclosed in the Hey-Shipton patent, conductive elements between non-adjacent capacitor pads in a multi-element lumped element filter are disclosed (see e.g., FIG. 13 of that reference). The linear arrangement of the resonators limits the number of elements realizable on a small substrate, while the phase requirements of the connecting line constrain cross-coupling. In addition, the Hey-Shipton patent does not disclose or teach any cancellation approach.
Therefore, one feature of the present invention is that it provides a method and apparatus for cancellation techniques to control the location of the transmission zeroes (or decrease the cross-coupling). Another feature is providing the use of a closed loop to enhance the cross-coupling. By providing means to increase or decrease cross-coupling, control over non-adjacent resonator device cross-coupling is accomplished, and transmission response of the filter is optimized.
In a preferred embodiment of the invention, in order to increase cross-coupling of non-adjacent elements, a closed loop coupling element is provided there between. In a second preferred embodiment of the invention, in order to decrease cross-coupling of non-adjacent elements, series capacitive elements are provided to cancel (or control) excessive inductive cross-coupling.
Therefore, according to one aspect of the invention, there is provided a filter for an electrical signal, comprising: at least one pair of non-adjacent resonator devices in a micro-strip topology; and a cross-coupling control element between the at least one pair of non-adjacent resonator devices, wherein transmission response of the filter is optimized.
According to another aspect of the invention, there is provided a bandpass filter, comprising: a plurality of L-C filter elements, each of said L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; a plurality of Pi-capacitive elements interposed between the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.
According to yet another aspect of the invention, there is provided a method of controlling cross-coupling in an electric signal filter, comprising the steps of: connecting a plurality of L-C filter elements, each of the L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; interposing a Pi-capacitive element between each of the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another; and inserting between the non-adjacent L-C filter elements a means for controlling cross-coupling between the non-adjacent L-C filter elements, wherein quasi-elliptical filter transmission response is achieved.
According to yet another aspect of the invention, there is provided a filter for an electrical signal, comprising: at least one pair of non-adjacent resonator devices in a micro-strip topology, wherein there is only a resonator device between the at least one pair of non-adjacent resonator devices; and a cross-coupling element between the at least one pair or non-adjacent resonator devices, wherein the transmission response of the filter is optimized.
According to another aspect of the invention, there is provided a method of controlling cross-coupling in an electric signal filter, comprising the steps of: connecting a plurality of L-C filter elements, each of the L-C filter elements comprising an inductor and a capacitor in parallel with the inductor; interposing a Pi-capacitive element between each of the L-C filter elements, wherein a lumped-element filter is formed with at least two of the L-C filter elements being non-adjacent one another and with only one L-C filter element between the two non-adjacent L-C filter elements; and inserting between the at least two non-adjacent L-C filter elements a cross-coupling element, wherein the transmission response of the filter is optimized.
These and other advantages and features which characterize the present invention are pointed out with particularity in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, the advantages and objects attained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described preferred embodiments of the present invention.